Evaluating Transformer-Based as well as Transformerless Uninterruptible Power Supplies

Choosing between transformer-based or transformerless uninterruptible power supplies may possibly not be a simple ‘either/or’ decision, particularly above 10kVA. Both technologies have their invest today’s power protection scenarios but the important thing differences between them are: physical size, efficiency, noise output and the levels of input harmonic distortion that they generate.

Transformer-based Uninterruptible Power Supplies: before early 1990s, the only design of online uninterruptible power was transformer-based. Nowadays, the style continues to be available but generally in larger sizes for UPS from eight to 800kVA. The most frequent applications for this are large industrial sites.

The inverter generates an ac supply from its dc power source, that will be fed right into a step-up transformer. The principal function of the transformer is to increase the inverter ac voltage to that particular required by the load. The transformer also protects the inverter from load disruption, whilst also providing Galvanic isolation (a method of isolating input and output).

Modern inverter designs use IGBTs  P2001 power station (Insulated Gate Bipolar Transistors) instead of more traditional switching components (such as power transistors and thyristors). IGBTs combine the fast-acting and high power capability of the Bipolar Transistor with the voltage control top features of a MOSFET gate to make a versatile, high frequency switching device. As a result has given rise to better, efficient and reliable inverters.

Transformer-based UPS will also be furnished with a double input option as standard, which may be selected at installation by removing a linking connector from its input terminal. This permits it to be powered from two separate ac supply sources thus adding further resilience. A transformerless UPS could be installed with dual input capability, with supplies derived from the exact same source, but that is typically a factory-fit option.

Transformerless Uninterruptible Power Supplies: transformerless UPS is a newer design, commonly available from 700VA to 120kVA. The principal purpose behind the introduction of transformerless units was to cut back the overall physical size and weight thus making an uninterruptible power unit more ideal for smaller installations and/or computer room/office type environments, where space might be limited. Additionally it generates far less noise and heat than its transformer-based cousin and has far lower input harmonic distortion levels making it suitable for environments where electronic equipment (such as computers) may be much more sensitive to this kind of distortion.

Rather than the step-up transformer, a transformerless UPS runs on the staged means of voltage conversion. The first stage combines a rectifier and booster-converter to generate a dc supply for the inverter. An uncontrolled, three-phase bridge rectifier converts the ac supply right into a dc voltage. That is passed through a mid-point booster circuit to step the dc voltage up to typically 700-800Vdc where a battery charger and inverter are powered. In the next stage, the inverter takes the supply from the booster-converter and inverts it back once again to an ac voltage to produce the load.

An additional advantage of this technique is that the rectifier can operate from whether three or single-phase input supply. This is configured at installation for systems up to 20kVA. A get a grip on system ensures a stable, regulated dc voltage is supplied to the inverter all the time and the inverter can operate irrespective of UPS output load variations or mains power fluctuations or disturbances.

Choosing between Transformer-based or Transformerless Uninterruptible Power Systems: in lots of applications the choice between both might be clear. It is where both ranges overlap, in terms of power rating, that the decision is more complicated. Consideration needs to be provided with then to: initial purchase cost, physical size, running costs, the installation environment, and particularly, the levels of input harmonic distortion they generate. Both designs could be operated in parallel to accomplish higher levels of availability and resilience.

Throughout the last decade, the gap between those two uninterruptible power technologies has reduced as manufacturers have applied common techniques and research & development efforts to both designs. The driving force behind it has been cost and size, alongside demands to enhance operating efficiency and reduce harmonic generation. When it comes to online performance, both designs provide the exact same level of performance and are classified as VFI systems (voltage and frequency independent – relating with EN/IEC 62040-3). Their principal differences are their effects on upstream supplies and the operating environment.

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